1. Technical Field of the Invention
The present invention relates to a reflecting mirror, the shape of which can be variably controlled by elastic deformation, and more particularly, a deformable mirror for laser beam having a high power density. The deformable mirror is contemplated to be used for beam propagation in a laser processing machine or in an oscillator, and optional adjustment or correction of the beam diameter at a focus point, the focus position and the beam mode can be conducted by controlling the shape of the reflecting mirror.
2. Description of the Related Art
Pure copper is generally used as the raw material of a deformable mirror for laser beam. This is because by using pure copper with good heat conduction, a part of the laser beam having a high power density is absorbed at the surface thereof and is emitted by heat conduction, thereby suppressing thermal strain.
Another reason that pure copper is used is because of its good workability in that it can be processed into a mirror finished surface having high accuracy of surface figures by diamond turning or polishing.
As techniques referring to a material of a deformable mirror for laser beam, the following can be exemplified.
(1) Japanese Patent Application Laid-open No. Hei 8-39282 teaches to use brass (an alloy mainly containing copper and zinc) having a fine structure disclosed in German Patent No. 3,710,334, which is a material that has high strength against so-called plastic deformation, which remains even after removing the force causing the deformation.
(2) In Japanese Patent Application Laid-open No. Hei 9-293915 a soft material having a Vickers"" hardness of 60 or less is used in consideration of its easy deformation.
A material for a reflecting mirror used as a deformable mirror for laser beam having a high power density must satisfy the following requirements (a) to (c), which enable a practical use.
(a) It has high heat conductivity, and thus part of a laser beam is absorbed at the surface thereof and is emitted by heat conduction, thereby suppressing thermal strain (high heat conductivity).
(b) It has a low Young""s modulus, and deformation of the reflecting mirror with pressure can easily occur (low Young""s modulus).
(c) It has high 0.2% yield strength. If the 0.2% yield strength (namely, stress at which plastic deformation of 0.2% is yielded.) of a material is high, it has higher restorative capability, namely, less plastic deformation, upon removal of the force causing the deformation. As a result, a wider range of controllable deformation can be obtained.
The conventionally proposed reflecting mirrors comprising brass are excellent in the 0.2% yield strength, but have low heat conductivity, and it is difficult to suppress the thermal strain to a small value.
In the invention disclosed by the present inventors in Japanese Laid-open Patent Application No. Hei 9-293915, copper, a copper alloy, aluminum and an aluminum alloy are exemplified as specific examples of a soft material having a Vickers"" hardness of 60 or less. However, these materials involve a problem in 0.2% yield strength, and have a narrow range where deformation control can be conducted. Thus, when it is deformed beyond the control range, it suffers plastic deformation.
The invention aims to provide a deformable mirror for laser beam that satisfies all the requirements (a) to (c) described above.
Because the material for a reflecting mirror used in a deformable mirror for laser beam must have both high heat conductivity and a low Young""s modulus at the same time, pure copper, pure aluminum or an alloy comprising copper or aluminum as a base material, to which a slight amount of an additive is added, is suitable therefor. By increasing the 0.2% yield strength of the material while maintaining high heat conductivity and a low Young""s modulus thereof to the extent possible, the requirements (a), (b) and (c) can be satisfied at the same time.
In order to attain this aim, the invention provides a deformable mirror for laser beam comprising a reflecting mirror having a shape that is variably controlled, the reflecting mirror being made of a copper alloy comprising copper having 3% by weight or less of magnesium and 1% by weight or less of phosphorous added thereto.
The copper alloy preferably comprises copper having 0.1 to 1% by weight of magnesium and 0.001 to 0.01% by weight of phosphorous added thereto.
As another means for increasing the 0.2% yield strength, aluminum, an aluminum alloy, copper or a copper alloy is used after being subjected to cold working or after being subjected to cold working and then heat treatment.
The invention therefore also aims to provide a deformable mirror for laser beam comprising a reflecting mirror having a shape that is variably controlled, the reflecting mirror comprising aluminum, an aluminum alloy, copper or a copper alloy having been subjected to cold working, or subjected to cold working and then heat treatment.
When the material used after being subjected to cold working or after being subjected to cold working and then heat treatment is a copper alloy containing magnesium and phosphorous as described above, the increase of the 0.2% yield strength can be fully achieved. In the case of the copper alloy containing magnesium and phosphorous, a working reduction of cold working is preferably 10% or more, and heat treatment temperature is preferably 600xc2x0 C. or lower.